A structure of transmitting a signal via communications between one piece of equipment and another piece of equipment which are mounted in a vehicle has been put in practical use, and many technologies about this structure have been also disclosed. As a conventional example of this structure, there is the following one.
This conventional example relates to equipment for vehicle which uses a micro computer (referred to as a “microcomputer” from here on) as a control unit for transmitting a signal to the other piece of vehicle-mounted equipment, and is characterized in that, in the equipment including a load which generates a large ripple current when a load current is alternately made to pass therethrough and prevented from passing therethrough, a switching element for performing an on/off drive controlling operation on the above-mentioned load according to an output signal from the above-mentioned microcomputer so as to make the load current pass through the load or prevent the load current from passing through the load, and a filter (choke coil) disposed between a low voltage terminal (which is a “circuit ground”) of the above-mentioned switching element, and a vehicle body ground, for reducing the ripple current so that, as a noise, the above-mentioned ripple current does not have a bad influence on the other piece of equipment, such as a radio, in order to eliminate the influence of a potential difference which occurs between the above-mentioned microcomputer (the vehicle body ground side) and the switching element (the circuit ground side) due to a voltage drop which occurs because of a large current flowing into the filter, a photo coupler is disposed between the microcomputer and the switching element as a signal transmission interface of insulated type which does not transmit any electric noise and the potential difference (for example, refer to patent reference 1).
In the above-mentioned conventional example, if the filter for reducing the ripple current is placed not on the side of the above-mentioned ground, but on the side of a positive power supply having a high potential, the low voltage terminal of the switching element can be connected directly to the vehicle body ground, i.e., the ground of the microcomputer, and the influence of the voltage drop as mentioned above (the potential difference) which occurs in the filter can be eliminated and therefore it becomes unnecessary to dare to use the photo coupler.
However, there is a case in which in order to suppress the occurrence of noise, the filter has to be disposed on the side of the ground, as mentioned above, and, in this case, the above-mentioned conventional example is needed in order to dare to place the filter on the side of the ground.
Furthermore, as an example having the same arrangement as that of the above-mentioned conventional example, except for the above-mentioned filter, there is an example in which a resistor for current detection is arranged between a switching element, which is controlled by a microcomputer, and a vehicle body ground. Also in this case in which the resistor for current detection is added, a potential difference occurs in the same way as mentioned above, and therefore the structure of the above-mentioned conventional example in which a photo coupler is disposed in order to eliminate the influence of the potential difference may be needed.
Furthermore, in order to prevent vehicle-mounted equipment from being destroyed when an abnormal condition like a reversal of the polarity of a battery power source (a reverse connection) occurs in an operating environment peculiar to the vehicle-mounted equipment, a diode for preventing a reverse current from flowing through the battery power source can be arranged in series with this battery power source. In this case, if the diode is placed on the side of the positive end of the battery power source, no voltage drop (no potential difference) occurs, unlike in the case of the above-mentioned conventional example. Furthermore, from the viewpoint of the structure of the diode, a frame to which the chip is mounted serves as the cathode of the diode in many cases, a direct connection of a case which can use, as a heat sink, the frame with a means of securing this frame thereto or soldering this frame thereto as a surface mounting component to the vehicle body ground eliminates the necessity of an electric insulating plate and so on, and the thermal resistance of a radiation path can be reduced. As a result, because heat generation of the diode element can be suppressed, this structure is convenient particularly to a diode through which a large amount of current passes and this result in heat generation and a high temperature. Thus, the structure of the above-mentioned conventional example can be an effective means for the case in which a diode is placed on the side of the vehicle body ground.
Furthermore, in equipment which allows a large amount of current to pass therethrough, because a diode which prevents a reverse current from passing through a battery power source at the time when a reverse connection of the battery power source is established has a voltage drop in the forward direction even under normal use, a loss caused by the diode, i.e., heat generation cannot be disregarded. In such a case, it is general to use a method of disposing an FET (field-effect transistor) instead of the diode, turning on this FET under normal use, and turning off the FET at the time when a reverse connection of the battery power source is established. In many cases, the FET having a low on resistance which is required in order to reduce the voltage drop at the time when the FET is turned on is of N-ch type. Furthermore, because FETs of N-ch type are cheap, this N-ch type of FET is used instead of the diode in many cases. Also in the case of using this N-ch type of FET, because a frame serves as the cathode (i.e., the drain of the FET) of a parasitic diode which the FET chip, like in the case of the above-mentioned diode, the FET of N-ch type is placed on the side of the vehicle body ground, too, and a direct connection of the frame to the vehicle body ground eliminates the necessity of an electric insulating plate and is convenient. In many examples, an FET of N-ch type is thus used.
As previously explained, the above-mentioned conventional example is effective for the case in which one of various elements of the various purposes, such as the filter (choke coil) and diode, is disposed between the switching element on the side of the circuit ground and the vehicle body ground.
Furthermore, in a case in which a microcomputer controls one load which allows a large current to pass therethrough by using one photo coupler while communicating with other vehicle-mounted equipment, as in the case of the above-mentioned example, if communication lines connected between the microcomputer and the other vehicle-mounted equipment consist of a transmission line and two reception lines via which a signal from this transmission line is received, i.e., if there is a one-to-one correspondence between the microcomputer and an object to be controlled, placement of a potential at which the microcomputer is mounted on the side of the vehicle body makes it possible to construct the circuit using one interface circuit such as a photo coupler, and is convenient.    [Patent reference 1] JP, 2003-154903,A
The conventional equipment for vehicle (patent reference 1) is constructed as mentioned above, eliminates the influence of the potential difference which occurs between the vehicle body ground side and the circuit ground side, which occurs when a large current flows into the filter for noise suppression, with a signal transmission interface which consists of a photo coupler, and carries out signal communications from the microcomputer to the switching element. Furthermore, the structure of conventional equipment for vehicle (patent reference 1) is not limited to a filter (a choke coil), and can also be applied to and effective for a case in which one of various elements of various purposes, such as a resistor for current detection, and a diode or FET for protection of reverse connection a battery power source, is placed between the circuit ground and the vehicle body ground.
However, because the above-mentioned conventional equipment for vehicle uses a photo coupler as a signal transmission interface, there is a one-to-one correspondence between the microcomputer and the object to be controlled.
In contrast to this, there is a case in which the microcomputer controls many loads (a plurality of loads) and so on as objects to be controlled. In this case, there is a one-to-many correspondence between the microcomputer and the objects to be controlled. When photo couplers are used for this structure in which here is a one-to-many correspondence between the microcomputer and the objects to be controlled, it is necessary to dispose a photo coupler for each of the objects to be controlled, and therefore the component count increases and the size of the equipment increases. As a result, there arises a problem that the cost of the equipment increases and this results in occurrence of an undesired situation.
Furthermore, because in general, communications for use in vehicle-mounted equipment are bidirectional communications in which transmission and reception are carried out via one communication line, there is another problem that only the structure of the above-mentioned conventional example which uses photo couplers cannot support such bidirectional communications, it is necessary to add other parts, and, as a result, the circuit structure becomes complicated and therefore it is not practical to use photo couplers.